Heat flux meters are used for measuring the heat flux densities going through walls, usually at quasi- steady state. The limitations of heat flux meters under dynamic conditions are well documented in the literature; nonetheless there is a theoretical limitation which is mostly not considered and should be also taken into account. Since heat transfer is a dissipative process, it would be expected to obtain transfer functions which act as low pass filters. Nonetheless, this paper shows that the transfer functions modeling heat flow rate may become high pass filters, which is against the physical evidence. In order to show this theoretical limitation of the heat flux meters, the heat equation is transformed in different classes of models, from partial differential equations to transfer functions related to surface temperatures and heat flux density going through a wall.